What a Casting Certificate Proves: Spectrometry, Hardness, Metallography
On incoming inspection there are only three tools for checking material: spectrometry, hardness and metallography. They prove different things, and missing any one of them leaves room for a well-written report to pass for something it is not.
1. Spectrometry fixes the composition, not the grade
Spark discharge atomic emission spectrometry is the usual method at the furnace and on goods-in. For cast iron it is GB/T 24234-2009, for carbon and low-alloy steel GB/T 4336-2016, for stainless steel GB/T 11170-2008.
What comes out is element content: carbon, silicon, manganese, phosphorus, sulphur, chromium, nickel, molybdenum. With castings there is a precondition. The sample has to be chilled to a white-cast structure first; otherwise graphite interferes with the discharge and carbon and silicon drift first.
More important, a composition inside the range does not make the grade compliant. A grade is a combination of composition and properties, and the same composition cooled differently can produce a different structure. Spectrometry can answer whether the wrong alloy went into the furnace. It cannot answer on its own whether the part is a conforming RTSi5.
2. Hardness reflects consistency of structure
Brinell hardness follows GB/T 231.1-2018, Vickers GB/T 4340.1.
On castings, hardness is mostly corroborating evidence. A wide spread across one batch or one location points to unstable pouring temperature or cooling. Low hardness usually means less pearlite in the matrix and coarse graphite. High hardness should raise the suspicion of chill or hard phases.
It does not prove heat resistance. Oxidation resistance and thermal-crack resistance above 800℃ depend on chromium and silicon content and on carbide stability; room-temperature hardness has no corresponding relationship with that curve. Using hardness as evidence of a temperature rating is the most common misuse.
3. Metallography exposes grey iron sold as ductile iron
Grey cast iron is examined under GB/T 7216-2023, ductile iron under GB/T 9441-2021, with sample preparation and general examination under GB/T 13298-2015.
Among high-silicon heat-resistant irons there is a pair that is easy to confuse: RTSi5 is grey cast iron and RQTSi5 is ductile iron. Their silicon targets are nearly the same, the spectrometry numbers come out close, and composition alone does not separate them. Metallography settles it at a glance: flake graphite means grey iron, nodular graphite means ductile iron.
This is the most practical step on goods-in, because the price difference between the two is real and the grade on the sheet is whatever the person filling it in chose to write. For how the grade system is arranged and which temperature each grade covers, see the material comparison table and the article on reciprocating grate bar grades.
4. Four items to write into the technical agreement
A single grade on its own is hard to argue with later. Put four things into the agreement:
- Composition: the grade and standard used for acceptance, and where the sample is taken (attached test block or the casting itself)
- Hardness: test method standard, number and position of measuring points
- Metallography: required graphite form and matrix, and the standard behind them
- Heat treatment or cooling method: the structure of a heat-resistant part is guaranteed by process, not by composition alone
With those four written down, a test report has a scale to be measured against. Giving them at the quotation stage is also more useful than arguing over a report afterwards.
